Power factor is defined as the cosine of the phase angle between the instantaneous alternating current (ac) voltage and current considering only the fundamental values of the ac voltages and currents. Unity power factor is achieved when the phase angle between the voltage and current is zero. It is desired to have unity power factor when drawing power from the utility so that only real power is drawn from the utility and no reactive power is drawn from the utility by. The investment on the generating and distribution equipments from the utility side will be minimized when only real power, having a power factor of unity, is drawn from the utility.
Power converts having rectifiers distort ac current from the utility supply, leading to non-sinusoidal current waveforms that introduce harmonics other than the fundamental that are undesirable in the operation of the utility. Additionally, these harmonics contribute to additional losses that do not exist when only sinusoidal currents are being drawn from the utility. Therefore, power factor correction and input current waveform shaping are of importance in motor drives applications, because of regulations and incentives from utilities to encourage users to build unity power factor operation into their systems so as to draw sinusoidal current from the utility. Common ways to incorporate these features are: (1) to add a separate unity power factor (UPF) correction circuit, which is an expensive approach and takes additional space and volume for installation, and (ii) to reuse a full-bridge controlled rectifier operating in a boost mode, which is an expensive solution.
A solution was patented by Krishnan Ramu (U.S. Pat. No. 7,271,564, Issued: Sep. 18, 2007) which addresses these challenges with a single transistor for a two phase machine. There are distinct disadvantages associated with the single-transistor power converter. One disadvantage is the limited torque generating region for a two phase machine. When phase A is conducting, phase B has to conduct, too. The torque production of these two phases are usually of opposite polarity most of the time. Therefore, the net torque production in such a circuit with two phase windings will have reduced output. A reduced torque outcome can be proven easily by looking at the instantaneous torque generation in two phases of the SRM.
An exception can be made such that that the currents in the two phases is unequal, so the torque contributions from the two phases are unequal in magnitude. The net torque is still reduced when the currents in the phases are unequal. Moreover, the reduced or smaller torque is produced in every alternate torque generation region of either phase A or B, whichever can produce the maximum torque compared to the other phase. Assume phase B has less winding turns and phase A has more turns. The unequal number of turns between the phases makes phase B the auxiliary phase, with smaller torque generating capability compared to phase A. Therefore, Tea-Teb, where Tea and Teb are the torque due to phase A and phase B of the SRM, is positive when phase A's torque generation region is positive. This net positive torque comes every alternate phase cycle thus average torque produced is halved, resulting in smaller torque output.
A circuit with two switches and two diodes per phase requires external boost inductors for each phase to address the problem of power factor correction. The external inductors are expensive, they require additional space in the motor drive system, and they require additional cooling to dissipate core and resistive winding losses. Most importantly, these external inductors receive current from the ac side but do not produce any useful torque, as they are not part of the electromagnetic system inside the SRM. The lack of useful torque provided by the external inductors is the biggest negative of the circuit and its operation.